1. Field of the Invention
This invention relates to a multi-layer film formed from at least two layers of dissimilar thermoplastic resins. More particularly, the invention is directed to a multi-layer film free of components corrosive to the processing equipment used to manufacture the film.
2. Description of the Prior Art
Multi-layer films made of polyamide and high density polyethylene (HDPE) are known in the art. For example, Olson, U.S. Pat. No. 4,220,684, discloses a coextruded laminated plastic bag employed to contain foods during storage and for reheating the foods prior to serving. The bag structure is comprised of two layers, the inner layer, in contact with the foodstuffs, formed from a high density polyethylene and the outer layer, in contact with the reheating vessel, formed from a relatively thin Nylon-6 film. The high density polyethylene layer also comprises a small amount of an adhesion promoter, such as ethylene-acrylic acid copolymers, wherein at least a portion of the acrylic acid groups are neutralized by an alkali metal ion. Such copolymers are commonly known in the art as ionomers.
Krieg, U.S. Pat. No. 4,401,256, also discloses a two-layer heat sealable film comprising a high density polyolefin containing a minor amount of an adhesion-promoting ionomer and a second polyamide layer.
Additionally, Ono et al, U.S. Pat. No. 3,819,792, Freshour et al, U.S. Pat. No. 3,496,061 and Smith, Great Britain patent application No. 2,045,774, also disclose the use of ionomer resins as adhesion-promoting agents for polyamides and high density polyethylene.
The ionomers were used in manfacturing such prior art multi-layer bag structures to promote the compatibility of the polyamide resin, e.g., Nylon, with the high density polyethylene resin. However, the salt-containing ionomers are very hygroscopic, cause foaming in the processing equipment, e.g., extruders, and promote corrosion of the processing equipment when the two layer films or bag structures made therefrom are manufactured.